Astronomy Today 9th Ed.

Review and Discussion

1. Name and briefly describe the main regions of the Sun.  How hot is the 
solar surface?  The solar core?
Refer to Figure 16.2 and Table 16.1 for help in answering this question. The regions are: 
a) core : where energy is generated through fusion of hydrogen into helium
   Tcore is about 16 million K.
b) radiative zone - a middle layer where energy from the core transfers outward by radiation (no convection) 
c) convective zone - an outer layer where convection is the primary source of 
energy transfer
d) photosphere - the "surface" of the Sun, where the gas is thin enough 
  for light to escape. Tsurf is about 5800 K.
e) chromosphere - a colorful layer of low-density gas above the photosphere
f) corona - the outermost atmosphere, which fades into the solar wind

The radius of the core is 200,000 km, the interior is 300,000 km thick, 
convective zone is 200,000 km thick, photosphere is only 300 km thick, 
chromosphere is 3,000 km thick, and the corona extends about a few million 
km above the chromosphere. The photosphere is at a temperature of 5800 K 
and the core is at a temperature of approximately 15 million K. 

2. What is luminosity, and how is it measured in the case of the Sun?
Luminosity is a measure of the total energy output of an object. It is the
energy in E-M radiation leaving in ALL directions.  For the Sun, it can be 
measured by experimentally determining how much solar energy is received by 
one square meter at the distance of the Earth from the Sun. This is then 
multiplied by the surface area of a sphere which encompasses the Sun and
whose radius is 1 AU.

3. How do scientists construct models of the Sun?
Knowing basic facts about the Sun, especially the fact that it is made 
primarily of light gasses such as hydrogen and helium, and knowing how such 
gasses behave under conditions of high pressure and temperature, allows 
astronomers to model the entire structure of the Sun. The model is correct 
if it successfully predicts observed properties of the Sun, such as its 
luminosity, radius, and temperature. Some of the input information to the 
model is uncertain, but the results suggest how correct this input data is. 
By making slight adjustments in the input parameters, the model is adjusted 
until its predictions are in agreement with all the observed properties. 
Once the model “works,” astronomers are then able to learn from the model 
about the properties in the interior of the Sun. Models are used as a test 
to see whether we fully understand the structure and processes of objects. 
They are also then used to predict properties that may not be directly 
observable. Models also make predictions of observables that help us further 
test the validity of the model. 

4. What is helioseismology, and what does it tell us about the Sun?
Helioseismology is the study of waves that ripple across the surface of 
the Sun. Some of these waves travel from deep inside the Sun. Their 
appearance on the surface provides information about the interior of the 
Sun that cannot otherwise be observed, such as temperature, density, and 
rotation speed.  In a similar manner seismic waves from the Earth can 
tell us about the Earth’s interior.

[5. How do observations of the Sun's surface tell us about conditions in the 
solar interior?
The oscillations observed on the solar surface are similar to seismic 
waves observed on Earth, although they are different in origin. The patterns 
of the waves are influenced by the internal structure of the Sun. Models 
of the solar interior predict how the waves should behave; observed 
waves suggest how the models need to be modified until there is agreement 
between observations and models. ]

6. Describe how energy generated in the solar core eventually reaches
the Earth.
The solar radiation is first produced in the core of the Sun, largely 
in the form of gamma rays. The gas in the core is totally ionized and
so the radiation passes through it fairly freely, except for scattering
off of electrons.  But as we get closer to the surface, the temperature 
drops, and more and more of the gas is not ionized or only partially 
ionized. Such a gas is more opaque to radiation. At the outer edge of the 
radiation zone, all of the radiation has been absorbed by the gas. This 
heats the gas and it physically rises, while cooler gas from the surface 
falls. This is the region of convection. The energy is transported by 
convection to the photosphere. Here, the density of the gas is so low that 
radiation can freely escape into space, and travel in a straight line to Earth.

7. Why does the Sun appear to have a sharp edge?
Virtually all the visible radiation we receive from the Sun comes from 
a thin layer called the photosphere. It is only 500 km thick; a small 
fraction of the Sun’s radius. The gas below the photosphere is too thick 
for light to escape, and the gas above is too thin to absorb and emit 
significant quantities of light.  Light can only escape from this narrow 
region, so the Sun appears to have a very well-defined edge as seen
from the Earth.  (500 km is small compared to the radius of the Sun, 
700,000 km.)  If it was viewed from a distance of 1000 miles, the
surface would not appear so sharp.

8. What is the solar wind?
Because the corona of the Sun is hot, some of the gas particles are 
traveling fast enough to escape the gravity of the Sun. The gas is mostly 
composed of the separated components of ionized hydrogen, protons, and 
electrons. This flow of high-speed particles away from the Sun is known 
as the solar wind. The escape of wind particles also depends on the Sun's
magnetic field structure: the solar wind eminates from "coronal holes"
where the field lines radiate away instead of looping back to the Sun.

9. Why do we say that the solar cycle is 22 years long?
Activity in the Sun’s magnetic field creates the cycle of sunspots seen 
on the Sun.  The solar magnetic field reverses itself every 11 years, so 
it takes 22 years – two “sunspot cycles” – to go through a single cycle of 
magnetic reversals. 

10. What is the cause of sunspots, flares and prominences?
All of these phenomena are caused by activity in the magnetic field 
of the Sun. Sunspots are caused by kinks or loops of magnetic field 
extending through the lower atmosphere. These areas of concentrated 
magnetic field repel hot material trying to rise up from the Sun’s 
interior, so the section of the Sun underneath the knot cools off and 
darkens.  Flares, by contrast, are areas where large amounts of energy 
are released in a short amount of time.  Their origin is mysterious, but 
they are somehow connected to instabilities in the magnetic field.  
Prominences are caused by material ejected from the Sun’s surface that 
follows along huge loops of magnetic field that carry the luminous gas 
far above the solar surface. 

11. Describe how coronal mass ejections influence life on Earth.
A coronal mass ejection (CME) is a cloud of ionized gas that travels 
quickly from the surface of the Sun outward, where it can be mostly 
deflected by the Earth’s magnetic field. Some of it gets channeled onto
the poles of the Earth, however, and ionizes the upper atmosphere creating
auroras.  This can produce low frequency EM radiation as well as the
pretty, visible glow.  It affects radio communications, and
can induce large currents/surges in electrical power grids, knocking them out. 
Earth satellites are also vulnerable to the Sun's outbursts (especially
flares, but also CMEs) because they are not protected by Earth's 
atmosphere and they have delicate electronics.

12. What fuels the Sun's enormous energy output?
Sun’s energy output is fueled by nuclear fusion of hydrogen 
into helium. In the process that takes place in the core of the Sun, 
4 hydrogen atoms (really just protons) come together and fuse to form 
a heavier element, helium. In this process, a small amount of mass is 
“lost.”  That missing mass has been converted into energy.  According to 
Einstein’s famous equation, E = mc2, a small amount of mass can become a 
large amount of energy.

13. What are the ingrediants and the end result of the proton-proton
chain in the Sun?  Why is energy released in the proton-proton chain?
A total of 6 hydrogen atoms go into the proton-proton chain. What 
comes out is a helium nucleus, two neutrinos, two positrons (which are 
quickly annihilated by colliding with electrons), energy in the form of 
gamma rays, and two hydrogens. Thus, only 4 hydrogen atoms are consumed 
to make the helium. The mass of helium produced by the nuclear fusion is 
0.7% less than the mass of the four hydrogens that were fused to make it. 
This small amount of “lost” mass is converted into energy. The amount of 
energy is easily calculated from E = mc2.

14. Why are scientists so interested in solar neutrinos?  What is the
most likely solution to the solar neutrino problem?
Neutrinos are produced in the proton-proton chain, which occurs in 
the core of the Sun. The neutrinos pass unimpeded through the Sun at 
nearly the speed of light. So neutrinos, in a sense, allow astronomers 
to directly observe the core of the Sun and the processes that occur 
there, almost as they happen. For a long time, astronomers were puzzled
because the Sun did not seem to be producing as many neutrinos as 
predicted.  There were two possible explanations for the low number of 
solar neutrinos received on Earth: either the Sun was under-producing 
neutrinos, or something was happening to the neutrinos as they traveled 
to Earth.  The first possibility was disturbing, as it would likely 
require the Sun’s core to be cooling by 10%.  But what could alter a 
neutrino across the void of space?  Recently, we have discovered that 
there are different kinds of neutrinos, and that they can transform into 
each other during the trip to Earth through “oscillations.”  By creating 
neutrino detectors that can detect all different kinds of neutrinos, we 
have confirmed that the Sun is producing all of the neutrinos we expect 
it to.

15. What would we observe on Earth if the Sun's internal energy source
suddenly shut off?  How long do you think it might take - minutes, days,
years, or millions of years - for the Sun's light to begin to fade? 
Repeat the question for solar neutrinos.
Light photons can take millions of years to fight their way out of the 
thick gasses in the Sun’s interior, while neutrinos fly out in a straight 
line at almost the speed of light.  Therefore, if we have a means of 
detecting neutrinos, we would know within minutes (8.3 min) if nuclear 
fusion in the Sun were to shut down.  If we only relied on visible light, 
however, it could take a million years before the Sun would start to dim.


Mult Choice.
1. c
2. b
3. c
4. a
5. a
6. b
7. b
8. c
9. c
10. b

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